The present invention relates to a glow plug used for preheating the interior of an auxiliary combustion chamber or a combustion chamber of a diesel engine. Specifically, the present invention is concerned with an improvement in a glow plug for a diesel engine provided with a ceremic heater which exhibits a function as a heater of the rapid heating type and provide a self-temperature saturation characteristic such that its heating characteristic can be improved to attain a long time after glow.
In general, since diesel engines have poor startability at a low temperature, there is employed a method to provide a glow plug within an auxiliary combustion chamber or a combustion chamber to heat the glow plug by passing a current therethrough, thereby to elevate an intake temperature or use it as an igniting source, thus to improve the startability of the engine. Hitherto, there have been ordinarily used glow plugs so called "sheath type plugs" wherein heat-resisting insulating powder is filled into a metalic sheath, allowing a coil-shaped heating wire of iron, chromium or nickel etc. to be embedded thereinto. In addition, glow plugs of ceramic heater type as shown in the U.S. Pat. No. 4,401,065 etc. have been known wherein there is used a rod-shaped heater formed by embedding a heating wire of tungsten etc. into a ceramic material such as a silicon nitride having insulating property. When compared with the glow plugs of the sheath type effecting indirect heating through the heat-resisting insulating powder and the sheath, such glow plugs of the ceramic heater type can provide improved heat transmission efficiency and excellent heating characteristic such that they are red-heated in a short time at the time of heating to improve temperature rising characteristic, thus exhibiting the performance as a heater of the rapid heating type. For this reason, such ceramic heater type glow plugs have been widely employed in recent years.
However, the drawbacks with these glow plugs of the ceramic heater type are as follows. They have a heater structure provided with the metallic heating wire (of tungsten) embedded into the insulating ceramic material e.g., silicon nitride etc., and thermal expansion coefficients of the both members are different from each other. As a result, they have a steep temperature gradient within the heater. There is the possibility that particularly rapid temperature rising at the time of heating and repetition of such a use would have bad influence upon durability of the ceramic heater, giving rise to problem in the reliability such as heat-resisting strength and high cost thereof.
To eliminate such problems, there has been proposed in the art a ceramic heater structure as shown in the Japanese patent pre-publication Nos. 60-9085 or 60-14784 etc. wherein a heating wire is formed with a conductive ceramic material having substantially the same as that of a ceramic material having insulating property. However, these ceramic heaters are questionable when used as a glow plug from structual and functional points of view, resulting in being beyond practical use.
For instance, the ceramic heater of the former has a structure provided with a conductive ceramic material serving as heating element embedded into an insulating ceramic material. Although this structure has a thermal transmission coefficient which is superior to that of the sheath type, it is questionable in exhibiting the function as the heater of the rapid heating type because of indirect heating. On the other hand, the ceramic heater of the latter is configured so that its heating element is exposed to the heater surface, thus making it possible to provide the function as the heater of the rapid heating type. However, since its heat element is formed as a stacked layer structure comprising simple U-shaped members and both ends of the heat element are simply guided to the heater rear end, the electrode leading-out structure becomes complicated, resulting in high cost. Further, the U-shaped heat element has a bad effect upon vortex flow into the combustion chamber, giving rise to problem in practical use.
Recently, glow plugs of this kind are strongly required in their market to employ so called "after glow system" for improving startability of diesel engine and durability with respect to the use at a high temperature resulting from the fact that the use of the diesel engine as a turbo engine is popularized to maintain the glow plug in an energized condition for a preselected time after the engine starts, thereby to smoothly and properly effect combustion within the engine, thus taking an exhaust and noise countermeasure. In addition, it is required to prolong this after glow time as long as possible (e.g., about ten minutes). To realize such a long time after glow, it is also required to self-control power delivered to the heat element to improve the heating characteristic to much extent to prevent overheat at the heater portion, and to have self-temperature saturation funtion to maintain the saturation temperature at a value less than a suitable temperature. Taking into account these matters, it is now expected to realize a glow plug provided with an inexpensive ceramic heater which has rapid heating property and self-temperature saturation characteristic etc., and which is excellent in regard to reliability such as heat-resisting strength etc.